The development of diffractive elements including diffractive lenses and mirrors allows the construction of a variety of large area optical devices, including telescopes, without the need for thick and heavy conventional optical components such as conventional glass lenses and mirrors. It is possible to construct telescopes for imaging or for collecting laser radiation using only a flat diffractive lens or mirror.
However, if the laser radiation to be collected is in the form of short pulses, such as would be the case for some laser communication systems or for systems designed for imaging, the use of diffractive components could result in stretching of the received laser pulse as a result of temporal dispersion of light received by the telescope. This result is undesirable in laser communication systems because it can interfere with the ability of the communication receiver to detect the pulse and to determine the location of the pulses, resulting in reduction of communication bandwidth and/or reduction of communication range.
Furthermore, in imaging using short-pulse laser illumination of the scene, stretching of the received pulses can degrade the accuracy of range measurements and/or reduce the range at which imaging can be performed with a given laser power.
Prior art methods exist for compensating chromatic aberrations in telescopes in which the primary optical element is diffractive. Examples of this prior art are provided in Andersen, G. et al., “Broadband antihole photon sieve telescope,” Applied Optics, Jun. 27, 2007, pp. 3706-3708, vol. 46 No. 18. In this work by Andersen et al., the primary optical element of the telescope was an anti-hole photon sieve, but the chromatic dispersion effects apply to any telescope with a diffractive primary element. Other prior art includes Early, J. et al.; “Twenty meter space telescopes based on diffractive Fresnel lens,” Proceedings of SPIE vol 5166, pp 148-156 (2004).
In this latter work, the primary collection optic was a 20-meter diameter Fresnel lens. In both the work of Andersen et al. and Early et al., correction for chromatic dispersion in the primary collection optic was provided by diffractive corrector elements, referred to in the paper by Andersen et al. as the “diffractive optical element,” and in the paper by Early et al. as the “Fresnel corrector.” Because the design concepts described in these two referenced prior art publications were intended to provide imaging over a relatively broad spectral band, the noted diffractive corrector elements provided correction over an optical bandwidth much broader than is required for optical communications. However, these prior art chromatic dispersion correction methods require multiple optical elements, in addition to the diffractive corrector elements. These additional optical elements add size, weight, and cost to the telescope system.
Antennas operating in the radio frequency and microwave bands, including bands in which the wavelength is between 1 mm and 10 cm, often are have a spherical or parabolic shape. The expense and weight of such systems could be reduced if the primary collecting element could be flat instead of curved, without introducing pulse stretching or other distortion in the received signals.
Hence, there is a need for a telescope system that includes a flat diffractive lens or mirror as the primary optic, and includes the means for eliminating temporal dispersion of received laser pulses or other received optical radiation or other electromagnetic radiation with a single optical corrector element.